Some gas turbine engines employ heat exchangers to elevate the temperature of the compressed air directed to the combustor and turbine sections of the engine. This heat for the incoming compressed air is provided by the hot exhaust gases that have already performed work by rotating the various arrays of turbine blades in the engine.
The heat exchange typically is carried out by passing the hot exhaust gases and the cooler compressed air on opposite sides of a thin sheet of metal. In such gas turbine engines, the heat exchanger can include a large number of such plates mounted in substantially face-to-face contact with one another. The plates are provided with undulations extending therethrough to define arrays of channels through which the exhaust gases and compressed air may flow. One desirable heat exchanger for a gas turbine engine is shown in U.S Pat. No. 4,431,050 which issued to John J. Martin on Feb. 14, 1984, and is assigned to the assignee of the subject invention. The heat exchanger shown in U.S. Pat. No. 4,431,050 includes a large number of very thin substantially identical annular plates which are stamped to include arrays of corrugations. The corrugations are formed to define the respective flow channels for heated exhaust gases or cooler compressed air on alternate sides of each plate shown in U.S. Pat. No. 4,431,050.
To properly channel the exhaust gases and the compressed air through the above described heat exchangers, it is necessary to provide secure and continuous attachments between selected plates. In annular heat exchangers, such as the heat exchanger shown in U. S. Pat. No. 4,431,050, these continuous and secure attachments must extend around both the inside diameter and the outside diameter of alternate adjacent plates to define attached pairs of plates. Although each such plate may be relatively small, (e.g. 12 inches to 28 inches), the plates are very thin (e.g. 0.005 to 0.010 inches) and will be stacked to define an axial length that may be at least as great as its diameter. In view of these geometric characteristics, there may be approximately one half mile of continuous attachments extending around the outer circumference of a stacked-plate heat exchanger of the type shown in U.S. Pat. No. 4,431,050. Similarly, this same heat exchanger may require approximately one quarter mile of attachments around the inside diameter.
The adjacent plates of the stacked-plate heat exchangers, such as the heat exchanger shown in U.S. Pat. No. 4,431,051, generally are secured to one another by welding. Attempts have been made to manufacture such devices by electron beam welding. In these attempts, individual strands of copper wire were wrapped circumferentially around the plates adjacent to the desired weld seam. An array of these plates then was placed in a vacuum chamber, and an electron beam was directed at the plate edges. The wire maintained contact between the plates during welding but was not welded to them. Although this technique worked well in experiments, it was totally unacceptable for production scale manufacture of heat exchangers. Specifically, the circumferential wrapping of wires was extremely costly, labor intensive and time consuming. Furthermore, the vacuum chamber also was costly, and attempts to rapidly feed arrays of heat exchangers into the vacuum chamber for welding would be unworkable.
In view of the problems encountered in electron beam welding of stacked-plate heat exchangers, virtually all such heat exchangers have been manufactured by resistance welding. Although the products produced by resistance welding have been acceptable, the manufacturing process has been slow and there have been several noticeable drawbacks. More particularly, the resistance welders utilize a pair of spaced apart rotating discs which function to both press the adjacent thin plates of the heat exchanger together and to carry an electrical current for fusing the two plates to one another. The two functions of these electrode discs tend to be irreconcilable. Specifically, the discs should be strong and highly abrasion resistant for holding the two plates against one another and for rotating relative to these plates. Ideally, a manganeze steel might be selected to perform this function. However, the electrodes must also carry a very high current, low voltage charge to effect the welding. Preferably a cathode copper or some similar highly conductive material would be selected to carry out this electrode function. Unfortunately, these highly conductive metals are not well suited to applying the pressure force to the plates. Consequently, it is necessary to make various compromises in selecting the electrode materials. Additionally, the weld produced by resistance welders is not readily visually observable. Consequently, the welds can only be spot checked by periodically cutting welded plates apart to ensure that the equipment is functioning properly. These spot checks are also very costly and time consuming. One resistance welder that has performed well is shown in U.S. Pat. No. 3,493,714 which issued to John J. Martin et al on Feb. 3, 1970 and is assigned to the assignee of the subject invention.
The thin sheets of metal used to form the heat exchangers also result in problems for the resistance welding apparatus. Specifically, the very thin sheets of metals used in these heat exchangers are likely to have a large number of burrs around their periphery. These burrs can interfere with the efficient resistance welding and therefore must be mechanically removed prior to welding. This separate mechanical preparation step also is time consuming, labor intensive and costly.
Laser welders have been used for various applications. Typically, the laser welder directs a tightly focused laser beam toward the joint to be welded. This laser beam causes a controlled amount of the adjoining surfaces to be melted and fused together. In developing the subject invention, tests were carried out using variations of the known laser welders. The known devices presented several significant problems when used to edge weld thin sheets of metal, as in the above described heat exchangers. Specifically, the above described sheets of metal were seldom perfectly planar. Consequently, the prior art laser welder could not be accurately directed to the actual location of the edges to be welded. Additionally, as noted above, the thin sheets of metal employed in the heat exchangers often have rough edges and burrs. When the laser beam was directed to this edge, the light comprising the laser beam would be reflected off the various irregular surfaces of the rough edge, thereby yielding an ineffective edge. Any effort to mechanically straighten the edges of these sheets and to eliminate the burrs were considered to offset the potential efficiencies of the laser welder.
In view of the above, it is an object of the subject invention to provide an efficient apparatus and process for welding the edges of two sheets of material.
It is another object of the subject invention to provide an apparatus and process for edge welding which employs a laser welder.
It is an additional object of the subject invention to provide an apparatus and process for welding the edges of very then sheets of metal to one another.
It is a further object of the subject invention to provide an apparatus and process for welding two irregular edges to one another.
Another object of the subject invention is to provide an apparatus and process for edge welding thin sheets of metal which eliminates the need to mechanically remove burrs and other edge irregularities prior to welding.